How to restrict a protocol to classes

There are some occasions when your protocol relies on reference semantics to work, which in practice means it can be adopted only by classes. For example, you might want to use the identity operator (===) to compare two instances of a conforming type, or you might want to change properties inside the type without relying on mutating methods.

To restrict your protocol in this way, make it inherit from AnyObject like this:

protocol Authenticatable: AnyObject {
func authenticate() -> Bool
}

Note: Some older Swift code uses class for this restriction, but AnyObject is correct for modern Swift.

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